In the aeronautical field, assemblies are required to undergo great stress and it is known that their stress-bearing capacity is directly linked to the distribution of compressive forces which are introduced at the time of installation, between the fastener means and the materials being assembled. The most critical zones are found at the level of the preformed head of the securing means and at the junction between the shank and the head The poorly-solved problem at the present time is that of obtaining, simultaneously, on the one hand a sufficient radial enlargement of the head to guarantee a good tightness between the head and the assembled materials, and on the other hand, a deformation and adequate expansion of the head toward the head/shank junction zone as well as in the shank in order to attain in these zones a higher level of compression restraints, thus assuring a satisfactory stress-bearing capacity.
In attempting to resolve this problem the head of the rivet has been provided with particular shapes, this head being flattened at the time of setting, for producing a flow of the material toward the sheets to be assembled; the adopted solutions heretofore have consisted of adding in projection on the frontal face of the head of the rivet protuberances or bulged parts which are then flattened at the time of placement and produce a greater flow than a flat head. Nevertheless, the shapes provided have a tendency to favor either the radial enlargement of the head to the detriment of the expansion at the level of the head/shank junction and of the shank, or in the reverse, the flow toward the shank to the detriment of the expansion of the head.
Thus certain fastener means are provided with a head having a bulged frontal surface (series of Dassault Standards 3.507.2 and 3.507.3 of 1968; 3.502.1 of 1966; FIAT Standards Norms 12830 of Feb. 5, 1971 intended for the MRCA aircraft; U.S. Pat. Nos. 4,000,680, 3,927,458, 3,747,467, 3,748,948) and the flattening of this bulged head increases the flow toward the shank, improving the compression forces therein; however, with the normal flattening forces, this surplus of material in the central portion of the head tends to reduce the radial enlargement of said head and to bring about a poor tightness between the head and the sheets. Other securing members include a head on which the surplus of material is provided on the periphery of the frontal surface of the head, in particular in the form of an annular projecting dome on this frontal surface (U.S. Pat. No. 4,051,592). During setting, this peripheral projection favors the radial enlargement of the head, bringing about a good tightness of the head in the sheets, however it is to the detriment of the flow toward the shank where the compression constraints remain weak, with the risk of rupture in the case of alternated constraints.